Radio frequency identification (RFID) systems have been used to track inventory, such as in a store. The inventory amount can be reduced as items containing RFID tags pass an RFID interrogator at time of purchase, for example. RFID tags can be either passive or active.
Passive RFID tags are interrogated with a radio frequency power source that bounces off of the passive tags, where the return signal uniquely identifies the tagged object with an RFID identifier (ID). In contrast, active RFID tags contain a power source (usually a battery) and can radiate an identifying radio signal, and are also known as RFID transmitter tags. A reader can receive and record such a signal, thus identifying the tagged object. Because passive tags have no power source, interrogating readers need to be in relatively close proximity to interrogate and read the passive tags. More expensive active tags, however, can be read from a greater distance, but which still has distance limits (such as within a few hundred meters depending on the power source).
Radio frequency ID tags are used in other industries as well. An RFID tag attached to an automobile during production can be used to track its progress through the assembly line. Pharmaceuticals can be tracked through warehouses. Even livestock and pets can have tags injected to facilitate positive identification of animals.